This invention relates to ultrasonic flowmeters and, more particularly, to method and apparatus for the ultrasonic measurement of the flow velocity of fluent media.
An ultrasonic measuring apparatus is known from German Specification 2,322,749, in which frequencies are generated which are a frequency f.sub.1 on downstream measurement related to the downstream speed of sound and a frequency f.sub.2 on upstream measurement related to the upstream speed of sound. From the instant of transmitting the ultrasonic signal, a predetermined number, for example 256, of output pulses of the oscillator are counted, whereupon a reference signal is transmitted. If it is found in a comparator that the ultrasonic signal received occurs earlier than the reference signal, the frequency of the oscillator is increased by means of a signal level generator in the form of an integrator, so that the timing of the reference signal is adjusted to the actual arrival time of the ultrasonic signal. If the ultrasonic signal received occurs later than the reference signal, the frequency of the oscillator is reduced. The frequencies f.sub.1 and f.sub.2 thus determined alternately are passed to a respective evaluating circuit, stored there until the arrival of the other frequency, and then processed to measuring data corresponding to the flow velocity or sonic velocity, respectively.
An apparatus of the kind referred to is known from U.S. Pat. No. 3,780,577, wherein a signal level generator is provided for both transmission directions, it being possible to transmit the early or late signals to the inputs of the signal level generator only if the associated logic elements simultaneously contain an upstream or a downstream signal, respectively. In this case, the signal level is also available in the respective other transmission direction during the measurement. By way of summation and subsequent integration of the signal level, one receives a voltage for controlling an oscillator of which the output frequency is selected as a measurement for the sonic velocity. The flow velocity is obtained by producing trapezoidal waves from the two signal levels. These trapezoidal waves are generated in a certain relationship to each other by means of switches, quick-acting integrators and comparators. In addition, the output frequency of the oscillator is stepped down to one quarter and modulated by the trapezoidal waves to produce the frequencies f.sub.1 and f.sub.2. The reference signal occurs after each 256 impulses of the frequencies f.sub.1 or f.sub.2.
Extraordinarily high requirements are placed on the accuracy of operation of these apparati because the transmission time differences are only 10.sup.-.sup.8 or 10.sup.-.sup.9 seconds for most applications. To achieve this accuracy, known apparati call for a considerable expenditure. This refers to, inter alia, certain circuit configurations, operational amplifiers of high quality and precision components. This tends to make the apparatus expensive.
The underlying object of the invention is to provide an ultrasonic measuring apparatus of the aforementioned kind having the desired measuring accuracy or even a greater measuring accuracy, particularly for the flow velocity, and which can be made considerably more cheaply as a result of using a simple circuit construction and normal components.